Retainer

Retainer is a slippery word because it covers several different commercial arrangements. A true retainer reserves a defined amount of the supplier's capacity (e.g. "up to 20 hours per month") and the client pays for that reservation whether or not they use it. An evergreen or pay-as-you-go retainer is closer to a recurring deposit: the client tops up a balance each month and the supplier draws against it as work happens. A scope-of-work retainer covers a specific recurring deliverable (e.g. "monthly content production") with a fixed monthly fee.

The commercial logic is predictability. The supplier gets reliable monthly revenue and can plan capacity around it; the client gets reserved access to the supplier without negotiating every engagement separately. The friction is the use-it-or-lose-it dynamic: a true retainer often does not roll over unused hours, which keeps the supplier's capacity planning clean but can frustrate clients who under-use in slow months.

Invoice the retainer in advance of the period it covers. A monthly retainer for May goes on the 1st of May (or late April) so the client pays before the supplier reserves capacity, not after. Pair the retainer invoice with a monthly statement showing hours used or work delivered, so the client can see the value before the next invoice arrives.